1. Field of the Invention
This invention relates to isotopically enriched helium-4 (helium-4, the helium-3 content of which is below that in natural gas source helium), to a method of producing said isotopically enriched helium-4 from liquefied natural gas source helium, and to an improved process for cooling a nuclear reactor with helium through the use of isotopically enriched helium-4 as the coolant.
2. Brief Description of the Prior Art
There are two natural sources of helium which have been used for the production thereof, namely, natural gas and air, and there are two naturally occurring helium isotopes, helium-4 and helium-3. The predominant isotope, helium-4, possesses an atomic weight of 4 and comprises all but about one part per million (ppm), or less, on a volume or mole basis, of natural source helium. This isotopic form of helium becomes a superfluid liquid (possesses zero frictional resistance) at temperatures below around 2.2.degree. K (obeys Bose-Einstein quantum statistics), and as a gas is impervious to radiation and possesses a low cross-section for neutron capture. Helium-3, the lighter isotope, has an atomic weight of 3, is present in natural source helium to the extent of about one ppm, or less, does not become superfluid at low temperatures (obeys Fermi-Dirac quantum statistics), and undergoes conversion in a strong radiation field through the absorption of beta particles to form tritium, the heaviest isotope of hydrogen, with a half-life of about 12-1/4 years.
The isotopic ratio of helium-3 to helium-4 in natural source helium is about an order of magnitude higher in helium derived from the air (atmospheric source helium) than in helium derived from natural gas (natural gas source helium). On occasion, in the past, helium has been extracted for needed purposes from the atmosphere, but today the exclusive economic source of helium is natural gas. In atmospheric source helium the helium-3 content is fairly uniform at about 1.2 ppm. In natural gas source helium, the helium-3 content varies somewhat, ranging, according to some of the older measurements of Aldrich and Nier (Physical Review, Dec. 1, 1948, p. 1590), from about 0.05 to about 0.5 ppm, depending on the location of the natural gas. These results are imprecise, with an admitted relative error of about 10-30 percent.
More precise measurements of helium-3 in natural gas source helium have recently been made, using high resolution mass spectrometry. These show the helium-3 content of various samples of natural gas source helium to run between 0.17 and 0.23 ppm, with a precision in the measurement of about 0.01 ppm, essentially an order of magnitude better than that of the older measurements of Aldrich and Nier. It is therefore now well established that natural gas source helium contains generally not less than about 0.17 ppm of helium-3.
Up until very recently the helium-3 content of atmospheric source or natural gas source helium was of academic or theoretical interest only. However, in the last few years, with the advent of nuclear power, one of the major nuclear designs which has emerged is that of the high-temperature gas-cooled reactor (HTGR), developed in the United States by Gulf General Atomic, Inc. The HTGR design uses helium gas as the coolant, to abstract heat from the nuclear core, which heat is then converted first into mechanical and then into electrical energy. The HTGR design has recently imparted considerable practical significance to the matter of the helium-3 content of helium, as will hereinafter be discussed.
Currently there are two HTGR reactors operating in the United States. The earlier one is the Beach Bottom plant of Philadelphia Electric Company, a pilot unit of 40 Mw(e), and the more recent one is the 334 Mw(e) Fort St. Vrain reactor of Public Service Company of Colorado. Orders have recently been placed with Gulf General Atomic for a half-dozen or so additional plants in the capacity range 770-1160 Mw(e), with an estimated value in the neighborhood of $2-3 billion.
The HTGR possesses a helium coolant inventory averaging about 2.0 million standard cubic feet (MMscf) of helium per 1000 Mw(e). It varies somewhat, ranging from about 2.6 for the smaller reactor sizes in the 350 Mw(e) class to about 1.4 for the largest reactor size of 1150 Mw(e). The helium circulates at an operating pressure of 350-700 psig, at a rate of about one-fourth of its inventory per hour. There is withdrawn for purification some 10 percent of the helium inventory per hour, and there is made up for mechanical and other losses some 10 percent of the inventory per year.
The advantage of helium over other coolants reposes in its low density and high heat capacity, with attendant lower circulating rates and power requirements; its ability to operate without thermal decomposition at very high temperatures (so far, up to 2400.degree. F), yielding high reactor thermal efficiencies; its chemical inertness (because it is a noble gas) toward any substance or component in the cirulating system with which it comes in contact; its stability, or imperviousness to radiation -- it itself is a product, as an alpha particle, of radioactive disintegration; and its low cross-section for neutron capture, giving good reactor neutron efficiencies. Further, helium is readily purified to a high degree, so that the impurities normally present in it -- nitrogen, neon, water, and hydrogen -- are in the very low ppm range.
Now that some operating experience has been gained with the HTGR, it has been discovered that the helium-3 content of the helium circulant is a detrimental impurity, in that it undergoes nuclear modification in the high intensity radiation field of the reactor and is converted into radioactive tritium, which hence needs to be removed continuously, along with radioactive fission products which may escape from time to time from the core into the helium coolant. These other radioactive contaminants which may enter the helium coolant are heavy inert gases such as krypton and xenon, and are not too difficult to remove in a purification system. Tritium, a hydrogen isotope, on the other hand, is much lighter and is therefore not readily removed by physical absorption onto charcoal; it must be oxidized and the tritium oxide adsorbed, or it must be removed by reaction with porous titanium metal sponge, which sponge must physically be replaced when exhausted. The disposal of the tritium, in whatever form eliminated, presents a radioactive solid waste problem, because of the relatively high level of radioactivity and long half-life of tritium.
Upon recognition of the helium-3 problem in helium circulant, operators of the newer HTGR plants have attempted to fill and operate their systems with helium containing the lowest possible amount of helium-3. Since, however, the only helium which has been available for any purpose, including nuclear reactors, is ordinary commercial helium derived from natural gas (natural gas source helium) containing, as described earlier, approximately 0.20 ppm of helium-3, these operators, in their recent purchases of helium, have specified sources containing helium-3 in the lower 0.17-0.18 ppm range, even though it would be desirable and advantageous for them to use helium with a helium-3 level an order of magnitude below this, which low helium-3 content helium simply does not exist in nature.
It can thus be seen how advantageous it would be to produce and have as an HTGR coolant isotopically enriched helium-4, or helium which is significantly lower in helium-3 content than exists in natural gas source helium and preferably below about 0.05 ppm, the benefits in HTGR use being related directly to the degree of reduction of helium-3 content below its level in natural gas source helium.
I have discovered that isotopically enriched helium-4, non-existent in nature and an improved coolant for high-temperature gas-cooled reactors, can be produced from liquefied natural gas source helium by the removal therefrom of helium-3.
Separations of helium-3 and helium-4 are in themselves not new in the art, but as heretofore practiced do not produce the desired material of the instant invention, or do not operate on the natural gas source helium feedstock necessarily required for the instant invention, or do not operate under the coditions of separation of the present invention. The prior art separation methods fall into three categories.
1. The source material is a mixture of 1-3 mol percent helium-3 in helium-4, argon, air, and traces of tritium, derived from U.S. Energy Research and Development Administration (ERDA) operations via the production of tritium by neutron bombardment of lithium-6 isotope, the tritium subsequently decaying to helium-3, which source material is separated for the express purpose of producing therefrom a relatively high purity helium-3. The enrichment process is that of thermal diffusion (Chemical Engineering, Nov. 25, 1963, page 64), carried out at the Mound Laboratory of Monsanto Research Corporation of Miamisburg, Ohio, operating under contract with ERDA. The products are helium-3 of 99 percent purity and residue gas with a helium-3 content of the order of 0.01 percent or 100 ppm.
A recent improvement on this enrichment process is the substitution for thermal diffusion of low temperature distillation under vacuum (pressure 130 mm), as described by Wilkes (Advances in Cryogenic Engineering, Plenum Press, Vol. 16, (1970), page 298). The column overhead operating temperature was 1.93.degree. K, the bottom temperature 2.80.degree. K.
2. Helium-3 enrichment is achieved using natural source helium as the starting material. One method is that of cryogenic gas centrifugation (Newgard et al., U.S. Pat. No. 3,251,542). Another method depends on the superfluid properties of helium-4 below around 2.2.degree. K to cause it to separate from helium-3 by passing through a superleak, the helium-3 being retained (Keller, Helium-3 and Helium-4, Plenum Press, page 36). The helium-3 enrichment achieved per pass is of the order of 5 times, with consequent low yields of enriched helium-3 and insignificant helium-3 denuding of the remaining helium-4. Successive passes of the helium-3 enriched product are more difficult and less successful because the lambda point (the temperature at which superfluidity is achieved) decreases with increasing helium-3 content.
A combination of superleak filtration and vacuum distillation, in that order, is employed by McKinney et al. in U.S. Pat. No. 3,421,334. A dilute helium-3 in helium-4 feed stream (residue gas from the above-described thermal diffusion process -- about 0.01 percent or 100 ppm of helium-3 in helium-4) is concentrated and purified to high purity helium-3 in a sequence of the two mentioned steps, in a common apparatus.
3. The feed material for the separation is about 6 mol percent helium-3 in helium-4. It is the heavier of two immiscible liquid phases in the helium-3, helium-4 dilution refrigerator, used to produce deep refrigeration, in the vicinity of 0.01.degree. K. Essentially pure helium-3 is removed from the feed by evaporation or pumping at about 0.7.degree. K under very high vacuum to regenerate the helium-3 refrigerant for recirculation.
Fairbank et al. (Physical Review, Vol. 71, pages 911-913, 1947) has reported coexistent vapor-liquid equilibrium data and relative volatilities for helium-3 and helium-4 in atmospheric source helium, at temperatures up to the normal boiling point of helium-4 (4.2.degree. K). The relative volatility for this system, at helium-3 concentrations in the liquid in the neighborhood of 1 ppm, characteristic of atmospheric source helium, diminishes from something in excess of 5 near the lambda point to something below 2 in the vicinity of the normal boiling point of helium-4, and it would be expected, as recognized by Fairbank that this relative volatility would become unity at the critical temperature of helium-4, namely 5.2.degree. K, at which point separation by distillation would become impossible. Fairbank did not employ natural gas source helium, with its significantly lower helium-3 concentrations than that of atmospheric source helium, and did not operate at superatmospheric pressure. It is well known that relative volatility in a non-ideal system, as this system is at these very low temperatures, is strongly dependent -- in an unpredictable manner -- on liquid concentration, and on system pressure.
Thus, so far as is known to me, none of the compositions described, produced, employed or encountered in the prior art is isotopically enriched helium-4, that is to say, helium with a helium-3 content significantly below the minimum 0.17 ppm level existing in natural gas source helium. Further, such separations of helium-3 from liquefied helium as have been made have occurred with a starting material no leaner in helium-3 than about 1 ppm, have produced enriched helium-3, and have been carried out generally under vacuum and at temperatures no higher than the normal boiling point of helium-4, under which prior art conditions the system is known to be characterized by high relative volatility and good separating power, although suffering from the economic disadvantages of high vacuum (low process throughput), high investment, and high operating (primarily refrigeration) costs.
Although higher operating pressures would generally be expected to be conducive to more economical vapor-liquid separations, this is not predictably so, particularly in the vicinity of the critical point of the system, because of an offsetting reduction in the relative volatility or separating power, for, as has been noted, at the critical temperature of the mixture the separating power disappears completely.
It was therefore surprising to find that the separation by distillation of helium-3 from liquefied natural gas source helium containing not more than about 0.23 ppm of helium-3 can indeed be effected, in temperature and pressure ranges beyond those previously attempted or employed, to yield isotopically enriched helium-4.